Gas turbine engines typically include a gas generator that defines an annular flow path and comprises, in an axial flow relationship, a compressor section for compressing air flowing along the flow path; a combustor section in which the compressed air is mixed with fuel and ignited to produce a high energy gas stream; and a turbine section that extracts energy from the gas stream to drive the compressor. Commonly many such engines also include a second turbine section known as a power turbine which drives a fan or propeller. One such type of engine known in the art utilizes a power turbine comprising alternately counter rotating blade rows that drive counter rotating fan blades.
Gas turbine engines generally, and their component parts in particular, are subject to vibrational stress and fatigue from a number of causes, including rotor rotational imbalance and pressure differentials within the engine. The vibrations can be so serious that the lifetime and integrity of a compressor or turbine casing, or the rotors and airfoils thereof, can be negatively impacted. Airfoil losses have occurred and in some instances, losses of pieces of airfoils have produced secondary failures on adjacent, downstream airfoils. While actual part failures are rare, structural damage to an engine may occur due to rubbing between vibrating engine parts. Such rubs are additionally undesirable due to the wear gaps, which can decrease engine performance, that are created between the rubbing parts. Therefore, because close tolerances between engine parts are required for good engine performance, minimization of engine vibration is desirable.
Vibrations are of greatest concern when the resonance frequency of the engine component part lies within the frequency range of the vibrations expected to occur during normal engine operations. Long, thin parts, for example airfoils such as low pressure compressor and turbine blades, vanes, and nozzles, and parts having a circular cross section such as rotors are of particular concern in this regard. Engine parts having circular cross sections are subject to nodal diameter vibration, a form of vibration characterized by two (or more in higher vibration modes) nodes on the circumference of the component part remaining stationary while parts therebetween oscillate.
Because of concern about the debilitating effects of gas turbine engine vibration, much industry time and effort has been devoted to the elimination or reduction of engine vibrations. Attempts to deal with vibrations have taken several lines of attack. One avenue of effort has taken the form of damping the component vibrations that do occur by the use of external means so as to keep them from reaching excessive levels of stress and deflection. Thus, turbine blades may be damped, for example, by the use of a "Z" interlock in the tip shrouds. That is, the vibrations are damped by the relative motion of the shrouds rubbing against one another such that the vibratory energy is absorbed. Another approach aimed at minimizing the problem is to increase the resonance frequency of a rotor, blade, or other component part by increasing the mass of the part, i.e., by making the part thicker and thereby stiffening it. The resonance frequency of the part is thereby increased so that it lies outside the vibrational frequency range expected in an operating engine. Increasing the mass of a part introduces excess weight and performance inefficiencies, however, and is not a desirable solution.
One method of stiffening that does not increase engine mass is to cast one or more parts as a unit. Thus, a compressor or turbine ring, including the appropriate airfoils and shrouds, can be cast as either a single unit or in large segments that are later joined together. Such large segment or full ring casting of an airfoil ring provides the necessary stiffness to increase the resonance frequency of the ring above anticipated nodal vibrational frequencies. In addition, this type of casting offers potential cost benefits; simplifies assembly of the compressor or turbine structure; and reduces engine weight, thereby increasing engine performance, by eliminating the apparatus otherwise needed to attach the airfoils to the ring. Nevertheless, while such casting stiffens the circular part, i.e., the ring, it does not provide for the damping of the airfoil oscillations that may occur, and, therefore, does not find ready application in present day gas turbine engines.